1. Field of the Invention
The present invention generally relates to a magneto optical disc, to which information is recorded by magneto optical recording, and an apparatus for reproducing the same. More particularly, the present invention relates to a magneto optical disc of high density recording type, to which information having a spatial frequency higher than a spatial frequency prescribed by the wavelength of a reproducing light and a numerical aperture of an objective lens at the time of reproduction, is recorded, and an apparatus for reproducing the same.
2. Description of the Related Art
Information cannot be reproduced from a magneto optical disc, if the information is recorded with such a high density that the spatial frequency of the recorded information is higher than a cut off frequency prescribed by the wavelength of a reproducing light and a numerical aperture of an objective lens at the time of reproduction.
In order to overcome the above mentioned problem, a super resolution reproduction such as a MSR (Magnetically induced Super Resolution) may be employed.
Here, the MSR is explained.
In the field of microscope, a resolution is improved by disposing an optical mask such as a pin hole (i.e. a physical mask) at the position of the object. The MSR is such a technique that, instead of disposing the physical mask at the medium plane of the magneto optical disc, a substantial mask is formed within the medium by use of the temperature distribution in the medium, so as to effectively increase the spatial frequency at the limit of reproduction. Thus, it is possible to improved the recording density by about 1.5 to 3 times (refer to SUPER RESOLUTION MAGNETO OPTICAL DISC, Japanese Applied Magnetic Academy issue, Vol.15, No.5.1991 etc.)
There may be propose various kinds of magneto optical discs, to which the MSR is applied. FIG. 13 shows a construction of an example of such a magneto optical disc.
As shown in FIG. 13, a magneto optical disc 1P is provided with: a reproducing layer P having a relatively small magnetic coercive force; a recording layer R to which information is recorded in a condition of vertical magnetization, and which has a relatively large magnetic coercive force; and a switching layer S for controlling the switched connection force between the reproducing layer P and the recording layer R.
Here, the reproducing operation of the above mentioned magneto optical disc is explained.
If the output power of the laser beam as the reproducing light is set to an appropriate value, a high temperature domain D.sub.H where the temperature is high, is formed at the back portion of the light spot as shown in FIG. 13.
When the temperature of the switching layer S at the high temperature domain D.sub.H, becomes more than the Curie point, the magnetic domain of the switching layer S is disappeared. Namely, the magnetic coercive force becomes zero, so that the switched connection force between the reproducing layer P and the recording layer R becomes small. At the same time, if the external reproducing magnetic field Hr is applied, the magnetization directions of small portions of the reproducing layer P which has a small magnetic coercive force, are all aligned in the magnetization direction of the reproducing magnetic field Hr.
Therefore, the high temperature domain D.sub.H becomes a mask layer, where the record information of the recording layer R cannot be read out. The record information of the recording layer R becomes information from the low temperature domain D.sub.L shaped in a crescent shape within the light spot. Thus, the size of the light spot can be substantially reduced, so that the information having a spatial frequency higher than the physical spatial frequency of the light spot, can be reproduced. Namely, the super resolution reproduction can be performed.
There is another type of MSR, in which the low temperature domain is used as the mask domain and the high temperature domain is used as the detection domain, on the contrary.
In this manner, in order to realize the MSR, two conditions are required i.e. (1) a condition that only the record information of the high temperature domain within the beam is read out or only the record information at the high temperature domain is masked within the beam, and (2) a condition that the record information is not erased during the reproducing process.
However, the above explained type of magneto optical disc needs at least two magnetic layers, in order to fulfil the above mentioned conditions (1) and (2). Further, since the composition margin of each layer is narrow, it is difficult to manufacture such a magneto optical disc that consists of two or more than two of magnetic layers in this manner.